Fuels from biomass, or so-called biofuels (such as bioethanol and biodiesel, for example) are promising alternatives to fossil fuels.
Biomass, a raw material for biofuel, includes saccharides (for example, starch) and oils and fats, and is produced by plants through photosynthesis. Accordingly, plants which are capable of active photosynthesis and intracellular accumulation of saccharides or oils and fats can be used as biomass sources. Corn and soybean are major plants that are currently used for biomass production. These crops are also consumed as food and forage, and dramatic increases in biofuel production would lead to soaring prices of food and forage, which has been disputed.
Under such circumstances, algae are attracting attention as alternative biomass sources to corn and soybean (see, for example, PTLs 1 and 2). Algal biomass production has advantages such as compatibility with food and forage supply and the massive algal growth.
For example, some mutants of Chlamydomonas, an alga, are known which lack a cell wall or have a thinner cell wall (cw15 and cw92, for example). These mutants have properties convenient for introduction of exogenous DNA into cells, and have been broadly used in gene transfer experiments. They are also helpful for increasing biomass productivity in that their cell is easily disrupted and facilitates recovery of contents thereof, and thus are reported to be used for biomass production. For example, PTL 3 discloses production of oils and fats using a cell-wall-deficient Chlamydomonas mutant. NPL 1 reports that a Chlamydomonas mutant with the cell wall mutation (cw15) and deficiency of a starch synthesis gene releases lipid droplets outside the cell. NPL 2 reports that a cell wall mutant of Chlamydomonas (cw15) further knocked out for a starch synthesis gene has increased productivity of oils and fats. NPL 3 is a known report on cell wall mutants of Chlamydomonas. 
PTL 4 reports a technique involving recovery of starch produced and extracellularly released by an algal source, Chlorella, and subsequent fermentation of the starch to produce ethanol. PTL 5 discloses a technique of modifying an alga to have an increased chloroplastic glutathione concentration for increasing its starch productivity.